Will Rogers said, “Things ain’t what they used to be and probably never was.” Yet nostalgia is the centerpiece of Donald Trump’s campaign to “Make American Great Again.”

Like his conservative views on defense, immigration and other issues, Mr. Trump threatens to reverse decades of carefully crafted environmental policy. As a Washington outsider and government critic, he shows no compunction about tearing up the environmental fabric that keeps our air and water clean, protects wildlife, reduces toxic wastes and improves quality of life. Clearly, making America great again does not include making America green again.

Mr. Trump asserts that environmental regulations are holding back America’s greatness. His energy agenda includes resurrecting the Keystone pipeline, lifting restrictions on oil and gas exploration and increasing drilling on federal lands. He is equally outspoken on coal, promising to revitalize Appalachia’s coal industry and put miners back to work. Never mind what more coal reliance would do to human health, forests and lakes or the fact that the coal industry’s decline is due to stiff market forces, notably cheaper and cleaner natural gas.

On climate change, Mr. Trump once tweeted that “the concept of global warming was created by and for the Chinese in order to make U.S. manufacturing non-competitive.” And he’s frequently claimed that climate change is “a hoax.” Such paranoia pulls the shades over science and the recent Paris climate accord between 191 nations, from which Mr. Trump promises to withdraw in addition to ending support for United Nations climate research.

The most telling sign of things to come is Mr. Trump’s call to abolish the “Department of Environmental,” by which he presumably means the non-cabinet level Environmental Protection Agency. “What they do is a disgrace; every week they come out with new regulations,” he’s said. When asked who will protect the environment, he replied, “We’ll be fine with the environment. We can leave a little bit, but you can’t destroy businesses.”

Not even conservative icon Ronald Reagan went this far. Reagan tried to defund pieces of the EPA, a strategy that backfired with mismanagement of the Superfund Program and hazardous waste cleanup. Within two years, Reagan had dismissed EPA Administrator Anne Gorsuch and recruited the smart and sensible Bill Ruckelshaus, EPA’s first administrator, to come back and restore the agency’s reputation.

While Mr. Trump may need to prove his conservative bona fides to Republicans in other ways, the environment is not among them. His candidacy is the inevitable outgrowth of the Republican Party’s half-century flight from environmental responsibility.

The GOP believes that its anti-science and anti-environmental stance is good politics, regardless of the health implications or collateral damage to the United States’ reputation around the world. It is hard to imagine that Republicans were once the party of Teddy Roosevelt, flamboyant in his own right, who waged a vigorous fight to protect the nation’s natural resources and wildlife for the benefit of future generations and that the Nixon Administration and Republican leaders like Bill Ruckelshaus helped to install the new pillars of U.S. environmental law that still guide us today.

Those succumbing to Mr. Trump have several coping mechanisms. They express faith that he will become more presidential and that his extremism will be checked by Congress. Some interpret his proclivity to change opinions as a sign of flexibility. And others excuse his incivility and self-promotion as mere entertainment, like his favorite boast, “We’re going to win so much you’re going to get sick of winning.”

Winning is the Trump mantra. He used it infamously against U.S. Senator and POW John McCain. It is also shorthand for Mr. Trump’s interest in freeing corporations from environmental requirements, enabling them to build more of what they want and where.

It will be a tough road ahead for the environment if Trump is elected. Effective environmental policies will be dismantled and U.S. environmental leadership will decline to the detriment of natural resource protection globally.

As a non-politician, Mr. Trump is transparent and direct. What he says is what he means. His backward-looking appeal, his intent to slash government and his extreme environmental views are real. The time is here for the environmental community to speak up against a looming catastrophe.